Recent progress has been made in the measurement of the electromagnetic characteristics of earth formations at radio frequencies. Such measurements are valuable in distinguishing fresh water bearing earth formations from oil formations in open well bore holes. For example, dual radio frequency measurements of dielectric constant and resistivity of bore hole media are disclosed in U.S. Pat. No. 3,891,916 which include the measurement of the amplitude of the total electromagnetic field at two different radio frequencies. These measurements are combined in order to simultaneously determine the resistivity and dielectric constant of the earth media surrounding a well bore hole. Also in U.S. Pat. Application Ser. No. 531,562 filed Dec. 11, 1974, and assigned to the assignee of the present invention, a technique for combining a radio frequency dielectric well log with a conventional deep induction well logging measurement in order to accurately determine the resistivity and dielectric constant of earth formation media in the vicinity of a well bore hole is disclosed.
While the radio frequency measurement of the dielectric constant and resistivity of earth formations in the vicinity of a well bore has proven to be very useful, particularly in areas wherein the well drilling fluid is fresh water or oil base mud, and where the earth formations surrounding the well bore are primarily fresh water filled or oil filled, these techniques have not proven to be entirely satisfactory in the case of saline drilling fluid filled well bores. Historically, the use of saline well drilling fluids has led to similar problems in the interpretation of conventional induction and resistivity well logging measurements. These problems occur in conventional electrical resistivity and induction logging measurements due to the invasion of highly porous earth formations by the drilling fluid from the bore hole. This bore hole fluid "invades" or penetrates the highly porous formations to some depth which is functionally related to the porosity, water and oil saturation of the formations and the permeability of these earth formations.
The effect of the invasion of conductive bore hole fluids into the invaded zone surrounding the bore hole in highly porous and permeable earth media leads to the masking of the true resistivity of the "virgin" or uninvaded formations in the vicinity of the bore hole. Because of this invasion problem, in the past conventional resistivity and induction logging apparatus has been designed in proliferation in order to be able to measure and distinguish the effects of the invasion fluid on the resistivity of the earth formations in the vicinity of the bore hole.
Focused induction logging instruments and resistivity measuring electrode instruments having different radial depths of investigation from the bore hole into the earth media surrounding the bore hole have been produced. Such instruments have been used in an attempt to measure the resistivity of the invaded zone (usually labeled R.sub.xo) and the resistivity of the virgin or uninvaded formation (usually labeled R.sub.t).
At the radio frequencies of interest with respect to radio frequency induction dielectric well logging the conductivity of the earth formations surrounding the bore hole is, in principle, intimately related to the measurement of the dielectric constant of the earth formation surrounding the well bore. Due to the possible "screening effect" of a conductive medium on the electromagnetic waves which are propagated from the transmitter coil of such a measurement system the amplitude of induced currents at the receiver coil or coils of such a system can be affected. If an amplitude measurement alone is relied on, as in the example of the previously mentioned patent, this screening effect can influence the received amplitude of the electromagnetic field at the receiver coils due to the attenuation of the transmitted signal. If the magnitude of the received signal is minimized by the effects of invasion, then it is apparent to one skilled in the art that it is more difficult to make the measurement with a small amplitude signal than it would be with a larger amplitude signal at the receiver coils.
Similarly, it the derivation of the theory of measurement of the dielectric constant at radio frequencies, it may be shown that both the resistivity and dielectric constant of the invaded earth formations affect the total field amplitude or relative phase shift of the electromagnetic field induced in the receiver coils by a transmitted signal from a radio frequency transmitter. Hence, both the resistivity and dielectric constant of the invaded zone must be accurately known in order to determine accurately the dielectric constant of the virgin formation earth material surrounding the bore hole. The foregoing patent and patent application offer alternative techniques for simultaneously determining both the resistivity and dielectric constant of the earth media in the vicinity of the bore hole. However, these techniques do not take into account the effects due to the invasion phenomena.